Dual double gate transistor and method for forming

ABSTRACT

The present invention provides a dual gate transistor and a method for forming the same that results in improved device performance and density. The present invention uses a double gate design to implement a dual gate transistor. A double gate is a gate which is formed on both sides of the transistor body. The present invention thus provides a transistor with two double gates in series that provide improved current control over traditional dual gate designs. The preferred embodiment of the present invention uses a fin type body with dual double-gates. In a fin type structure, the double gates are formed on each side of a thin fin shaped body, with the body being disposed horizontally between the gates.

This application is a divisional of Ser. No. 10/063,323; filed on Apr.11, 2002, now U.S Pat. No. 6,750,487.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to the field of semiconductormanufacturing and, more specifically, to a method for forming doublegated field effect transistors.

2. Background Art

The need to remain cost and performance competitive in the production ofsemiconductor devices has caused continually increasing device densityin integrated circuits. To facilitate the increase in device density,new technologies are constantly needed to allow the feature size ofthese semiconductor devices to be reduced.

The push for ever increasing device densities is particularly strong inCMOS technologies, such as the in the design and fabrication of fieldeffect transistors (FETs). FETs are used in almost all types ofintegrated circuit design (i.e., microprocessors, memory, etc.)Unfortunately, increased device density in CMOS FET can result indegradation of performance and/or reliability.

One type of FET that is used in many different applications is a dualgate FET. A dual gate FET has two separate gates in series that are usedto control the operation of the FET. For example, in a dual gate NFET,both gates must be high for the transistor to be on. If either gate islow, the transistor does not turn on. Likewise, in a dual gate PFET,both gates must be low for the transistor to be on. If either gate ishigh, the transistor will not turn on.

Dual gate FETs have many different uses today. Dual gate FETs are usedin logic operations, effectively providing a logical NAND operation withone device. Dual gate FETs are also commonly used in radio frequencyapplication where the dual gate structure can be used to provideadditional isolation between the drain and the gate. This reduces thepossibility of capacitive feedback from the drain back to the gate, andthus improves operation of the device.

Unfortunately, several difficulties arise in the design and fabricationof dual gate transistors. For example, it has been difficult to designdual gate transistors with sufficient device density and current controlability when compared to single gate transistors. This has limited theirapplication to low performance applications.

Thus, there is a need for improved device structures and methods offabrications of dual gate devices that provide improved deviceperformance and device density.

DISCLOSURE OF INVENTION

Accordingly, the present invention provides a dual gate transistor and amethod for forming the same that results in improved device performanceand density. The present invention uses a double gate design toimplement a dual gate transistor. A double gate is a gate which isformed on both sides of the transistor body. The present invention thusprovides a transistor with two double gates in series that provideimproved current control over traditional dual gate designs. Thepreferred embodiment of the present invention uses a fin type body withdual double-gates. In a fin type structure, the double gates are formedon each side of a thin fin shaped body, with the body being disposedhorizontally between the gates.

In another aspect of the invention, the dual double gates are providedwith offset gaps between the double gates. The offset of the gaps allowsthe size of the gaps to be increased, reducing gate to gate capacitance,while still providing exceptional current control.

The foregoing and other advantages and features of the invention will beapparent from the following more particular description of a preferredembodiment of the invention, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

The preferred exemplary embodiment of the present invention willhereinafter be described in conjunction with the appended drawings,where like designations denote like elements, and

FIG. 1 is a top cross sectional view of a dual double-gate transistor;

FIG. 2. is a flow diagram illustrating a fabrication method;

FIGS. 3–10 are top and cross-sectional side views of an exemplary dualdouble gate transistor during fabrication; and

FIG. 11 is a top schematic view of a dual double-gate transistor withoffset gates.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention provides a dual gate transistor and a method forforming the same that results in improved device performance anddensity. The present invention uses a double gate design to implement adual gate transistor. A double gate is a gate which is formed on bothsides of the transistor body. The present invention thus provides atransistor with two double gates in series that provide improved currentcontrol over traditional dual gate designs. The preferred embodiment ofthe present invention uses a fin type body with dual double-gates. In afin type structure, the double gates are formed on each side of a thinfin shaped body, with the body being disposed horizontally between thegates.

In another aspect of the invention, the dual double gates are providedwith offset gaps between the double gates. The offset of the gaps allowsthe size of the gaps to be increased, reducing gate to gate capacitance,while still providing exceptional current control.

Turning now to FIG. 1, a top sectional view of an dual double gatetransistor 50 is shown. The dual double gate transistor 50 includes abody 52, a first gate 54, a second gate 56, gate dielectrics 58,source/drains 60 and spacers 62. The transistor 50 is a dual double gatetransistor because both the first gate 54 and the second gate 56 areformed on two sides of the body 52. Specifically, a first portion ofgate 54 is formed on one side of the body 52, and a second portion ofgate 54 is formed on another side of body 52. Likewise, a first portionof gate 56 is formed on one side of the body 52, and a second portion ofthe gate 56 is formed on another side.

The dual double gate transistor 50 is a fin transistor, meaning that thetransistor body is from a thin upright portion of semiconductor and thushas a “fin” type shape. In a fin transistor, the gate(s) are formed onthe side(s) of the fin body and the source and drain are formed at theends of the fin body. In a fin-type body, the fin is generally narrowenough to allow the channel to become fully depleted during operation.It should be noted that as this is a cross sectional view, the topportions of gates 54 and 56 are not shown. Typically, both halves ofeach gate are connected at the top of the transistor, although the twogates are not connected to each other.

The dual double gate transistor 50 is formed such that a first gate andthe second gate are in series. Thus, where the transistor is a NFET bothgates would have to be high to turn on the transistor and where thetransistor is a PFET both gates would have to be low. This allows thetransistors to implement logic functions such as a logical NANDoperation. It also allows for increased drain-to-gate isolation bysetting an intervening gate at a fixed voltage level.

One key parameter of the dual double gate transistor 50 is theseparation between the first gate 54 and the second gate 56. Generally,it is desirable to provide sufficient distance between the gates toassure that capacitance between the gates is minimized. However, theamount of a distance should not be so great as to create a large “dead”space, with no gate coverage, in the middle of the transistor body. Aswill be discussed with reference to FIG. 11, a second embodiment is alsoprovided that provides that mitigates the need for placing the gatesclose together to prevent dead space.

Turning now to FIG. 2, a method 100 for forming a dual double gatedtransistor in accordance with the preferred embodiment is illustrated.As will become clear, method 100 can be used to form a fin type dualdouble gated transistor similar to that illustrated in FIG. 1. Anexample of a transistor being formed with method 100 will be illustratedin FIGS. 2–10. Of course, those skilled in the art will recognize thatthe principles of the present invention apply to other designs andmethods for fabrication. For example, the method will be described withthe reference to the formation of a fin type transistor, and thoseskilled in the other will recognize that it can be applied to theformation of a other types of double gate transistors, such as planardouble gate transistors.

The first step 101 of method 100 is to provide an appropriate wafer. Inthe preferred embodiment, the wafer used comprises a silicon oninsulator (SOI) wafer. As such, the wafer comprises a buried oxide layerbeneath an SOI layer. As will become clear, the SOI layer is used toform the body of the double gated transistor. As such, it is generallypreferable to use a SOI layer that has a p-type (for NFETs) dopingdensity in the range of 1×10¹⁸ cm⁻³ to 8×10¹⁸ cm⁻³ to provide propercentering and control of the threshold voltage of the transistor. Inother cases, the proper doping will be provided later using anappropriate implant into the fin.

However, non-SOI wafers can be used. When a non-SOI wafer is used, theprocessing remains otherwise identical to those of the SOI wafer case,except as noted.

Turning now to FIGS. 3A and 3B, a wafer portion 200 is illustrated. FIG.3A illustrates a top view of the wafer portion 200, and FIG. 3Billustrates cross-sectional side views taken at line 3B. The preferredembodiment wafer portion 200 comprises an SOI wafer, and as suchincludes an SOI layer 202 and a buried oxide layer 204.

Returning to FIG. 2, the next step 102 is to pattern the SOI layer todefine the fin. When forming fin type transistors, it is generallydesirable to have the fin thickness narrow when compared to the gatelength. Typically, the fin thickness should be less than one quarter ofthe gate length to give good threshold voltage control. Also, it isgenerally desirable that the fin thickness should be greater than 2.5 nmto avoid degraded mobility due to quantum confinement issues. As thegate length is generally made to minimum feature size, image enhancementtechniques are desirable to achieve a fin width of subminimum featuresize. One such image enhancement technique is sidewall image transfer.

Sidewall image transfer typically involves the formation of a mandrellayer and at least one etch stop layer. The mandrel layer is thenpatterned to define shapes such that the exterior perimeter of theshapes will provide the sidewalls used to define the fin. The mandrellayer preferably comprises a layer of oxide, nitride or other suitablematerial. Generally it will be desirable for the mandrel layer to have athickness of between 10 nm and 100 nm, however, such a thickness maychange depending on the desired body thickness.

Sidewall spacers are then formed on the sidewalls of the patternedmandrel layer. The sidewall spacer can be formed using a deposition ofsilicon nitride or other suitable material, followed by a suitabledirectional etch.

The SOI layer is then etched selective to the sidewall spacer to definethe fin. This can be done using any suitable etch that is selective tothe sidewall spacer. This etch transfers the trimmed sidewall spacerimage into the SOI layer, and thus completes the sidewall imagetransfer. This is preferably done by using a reactive ion etch thatetches the SOI layer selective to the sidewall spacers and stops on theburied insulator layer.

Sidewall image transfer provides many advantages, the most notably beingthat it allows features to be accurately fabricated below the miniumlithographic feature size. Specifically, because sidewall image transferis used to define the fins, the fins of the transistor can be accuratelyformed with a narrower width than could be formed using traditionallithography.

Turning now to FIGS. 4A and 4B, a wafer portion 200 is illustrated afteran etch stop layer 208 and a mandrel layer 212 has been formed on theSOI layer 202, mandrel layer 212 has been patterned, and a sidewallspacer 213 formed on the edge of the mandrel layer 212. FIG. 4Aillustrates a top view of the wafer portion 200, and FIG. 4B illustratescross-sectional side views taken at line 4B. Turning now to FIGS. 5A and5B, a wafer portion 200 is illustrated after the sidewall spacer 213 hasbeen used to etch the SOI layer 202 to form a fin body 214. Again,because the fin body 214 has been defined using sidewall image transfer,it can accurately formed with a subminimum feature width.

Returning to FIG. 2, the next step 103 is to perform a sacrificial oxideprocess to clean the exposed sides of the fin. This would typicallyinvolve growing a thin layer of thermal oxide and then removing theoxide. This process removes defects in areas of the fin that weredamaged by the patterning process, and prepares it for the formation ofthe gate dielectric.

In some cases it will be desirable to perform a ion implant into thebody at this time. Generally, an ion implant is performed after theoxide has been grown but before it is removed. This ion implant servesto properly dope the fin channel region. In other cases, the fin channelis doped in situ during the formation of the fin material.

The next step 104 is to provide gate oxide or other dielectric on theexposed surfaces of the silicon by either thermal oxidation of theexposed silicon or deposition of silicon dioxide or high k dielectric.This process forms gate dielectric on both sides of the fin. The nextstep 106 is to deposit and pattern a first gate conductor material. Thisdeposition and patterning forms the first of the two double gates.

The gate conductor can be any suitable conductive material such as dopedpolysilicon or tungsten. Generally, the gate conductor material isdeposited over the fin, planerized, an insulator layer deposited, andthen the gate conductor is patterned using a suitable lithographicprocess. The insulator layer formed on the first gate conductor willserve to isolate the first gate conductor from the second gateconductor. Additionally, the insulator layer can be used as a hardmaskduring the patterning of the first gate conductor.

Turning now to FIGS. 6A and 6B, the wafer portion 200 is illustratedafter the forming of a gate dielectric, the deposition of a first gateconductor, the deposition and patterning of an insulator layer, and thepatterning of the first gate conductor. FIG. 6A illustrates across-sectional top view of the wafer portion 200 taken at line 4A, andFIG. 6B illustrates cross-sectional side views taken at line 4B. Thedeposition and patterning of the first gate conductor forms insulatorlayer 217 and a first gate 216 over the gate dielectric 218. Theinsulator layer 217 covers the first gate 216, and will be used toisolate the first gate 216.

Returning to FIG. 2, the next step 108 is to perform sidewallreoxidation and form a first gate spacer. The sidewall reoxidationserves to remove damage from the sides of the fin that may have occurredduring the patterning of the first gate structure. The formation of thefirst gate spacer provides the needed isolation between the first gateand the later formed second gate. The first gate spacer can be formed bythe conformal deposition and directional etch of a suitable dielectric,such as silicon oxide or nitride. This forms the gate spacer on thesidewalls of the first gate.

Generally, the first gate spacer should be constructed exclusively onthe gate, and not on the sides of the fin. This can be accomplishedusing a method such as described in U.S. Pat. No. 5,793,082, issued toBryant, and assigned to International Business Machines, Inc. In thismethod, a dielectric material is deposited over the entire wafer andplanerized to a level where the top of the gate is exposed. A selectiveetch is next used to recess the dielectric below the top of the gate,typically 5 nm to 20 nm. Next, a nitride spacer is formed on the exposededges of the gate and subsequently used as a mask to etch the depositeddielectric, thereby forming a spacer adjacent to the gate edge only.

Turning now to FIGS. 7A and 7B, the wafer portion 200 is illustratedafter a first gate spacer 219 has been formed around the first gate 216.

Returning to FIG. 2, the next step 110 is to again perform a sacrificialoxide process to clean the exposed sides of the fin, and to regrow thegate oxide. The sacrificial oxide process again involves the growing ofa thin layer of thermal oxide and then removing the oxide. This processremoves defects in areas of the fin that were damaged by the patterningprocess, and prepares it for the formation of the oxide. The gate oxidecan then be regrown using a suitable thermal oxidation process. Thisprovides the needed gate dielectric on both sides of the fin inpreparation for the formation of the second gate.

In some cases it will again be desirable to perform a ion implant intothe body at this time. Generally, this ion implant would be performedafter the oxide has been grown but before it is removed. This ionimplant serves to separately adjust the channel threshold voltage forthe second gate in cases where that is desirable.

The next step 112 is to deposit and pattern the second gate conductor.This deposition and patterning forms the second of the two double gates.Again, the gate conductor can be any suitable conductive material suchas doped polysilicon or tungsten. Generally, the second gate conductormaterial is deposited over the fin, planerized, a hardmask deposited andpatterned using a suitable lithographic process, and then patterning thesecond gate conductor. The previously formed first gate spacer and firstinsulator layer provides the isolation between the first gate and thesecond gate.

Turning now to FIGS. 8A and 8B, the wafer portion 200 is illustratedafter the reforming of a gate dielectric and the deposition andpatterning of a second gate conductor. The deposition and patterning ofthe first gate conductor forms a second gate 220 over the reformed gatedielectric 218. A portion of the hardmask layer 223 remains over the topof the second gate 220. The previously formed first gate spacer 219 andinsulator layer 217 provides the isolation between the first gate 216and the second gate 220.

Returning to FIG. 2, the next step 114 is to perform a sidewallreoxidation and then form a second gate spacer. The sidewall reoxidationagain serves to clean the sides of the fin that may have been damagedduring the patterning of the first gate structure. The formation of thesecond gate spacer isolates the second gate. The second gate spacer canbe formed by the conformal deposition and directional etch of a suitabledielectric, such as silicon nitride.

Turning now to FIGS. 9A and 9B, the wafer portion 200 is illustratedafter a second gate spacer 221 has been formed around the exposed edgesof the second gate 220.

Returning to FIG. 2, the next step 116 is to form source/drain implantsinto the exposed outer portions of the fin. This is preferably done byperforming an angled implant into the source and drain regions of thefin. The angled implants preferably comprise arsenic for n-type FETs orboron difluoride for p-type FETs, tilted between 45 degrees and 75degrees from a ray normal to the plane of the wafer. The doses andenergies of the these implants preferably range from between 2×10¹⁴ to1×10¹⁵ cm⁻² at 0.5 to 5 keV. Turning now to FIGS. 10A and 10B the waferportion 200 is illustrated after the formation of source/drain implants223 into the fin body.

Returning to FIG. 2, the last step 118 is to complete the transistors.This would typically include the formation of contacts and anyinterconnect processing. These contacts would typically include sourceand drain contacts, and contacts to both the first gate and the secondgate. Contacts to the first and second double gates allow the two doublegates to receive control signals and be independently activated tocontrol the operation of the transistor.

Method 100 thus provides a method for the fabrication dual double gatedtransistors that provides a dual gate transistor with improved densityand performance.

Turning now to FIG. 11, top sectional view of a second embodiment of thedual double gated transistor is illustrated. The dual double gatetransistor 51 includes a body 52, a first gate 54, a second gate 56,gate dielectrics 58, source/drains 60 and spacers 62. The transistor 51is a dual double gate transistor because both the first gate 54 and thesecond gate 56 are formed on both sides of the body 52. Again, it shouldbe noted that as this is a cross sectional view, the top portions ofgates 54 and 56 are not shown. Typically, both halves of each gate areconnected at the top of the transistor, although the two gates are notconnected to each other.

As stated before, one key parameter of the double gate transistor is theseparation between the first gate 54 and the second gate 56. Generally,it is desirable to provide sufficient distance between the gates toassure that capacitance between the gates is minimized. However, theamount of a distance should not be so great as to create a large “dead”space, with no gate coverage, in the middle of the transistor body. Thesecond embodiment transistor 51 mitigates the problem of dead space byoffsetting the space between the gates such that they do not line upacross the fin. In particular, because the space is offset, there is nolocation in the body 52 of the transistor that is not adjacent to eitherthe first gate or the second gate. Because the body is thin in a fintransistor, the single half the gate can provide a sufficient currentchannel for reliable operation. Because the problem with dead space hasbeen mitigated, the space between the first gate 54 and the second gate56 can be larger, reducing the capacitance between the gates.

The dual double gate transistor 51 can be formed using method 100 byforming the first gate at a non-perpendicular angle with respect to thefin. By patterning one edge of the first gate at an angle other than 90degrees, preferably between 45 and 60 degrees, the gate on one side ofthe body will extend between 1.4 and 2 times the body thickness beyondthe gate on a second side. Thus, when the second gate is formed, thesecond gate on the second side will overlap, within one body thickness,the first gate on the first side of the body, ensuring good continuityof control of the body potential by the gates, collectively.

It should also be noted that the gate electrodes on opposing sides ofthe transistor body may be either of the same material, such as n-typepolysilicon, p-type polysilicon, tungsten, titanium nitride, rhenium, orother conductors or of differing materials selected from a similar list.The second gate electrode pair may also be of materials different fromthose comprising the first gate electrode pair. When such dissimilarmaterials are used in the first gate or in the second gate, that side ofthe gate with material properties which render it more attractive to thedominant charge carriers (e.g. n+ polysilicon versus p+ polysilicon foran n-type FET constructed of asymmetrically doped polysilicon gates) isreferred to as the strong gate while that side of the gate which is lessattractive to the channel carriers is referred to as the weak gate. Inthe case when dissimilar materials comprise the first gate electrodepair and furthermore, dissimilar materials comprise the second gateelectrode pair, it is generally favorable to construct thedual-double-gate transistor with a portion of the strong gate side ofthe first gate directly opposite the body from a portion of the stronggate side of the second gate.

The present invention provides a dual gate transistor and a method forforming the same that results in improved device performance anddensity. The present invention uses a double gate design to implement adual gate transistor. A double gate is a gate which is formed on bothsides of the transistor body. The present invention thus provides atransistor with two double gates in series that provide improved currentcontrol over traditional dual gate designs. The preferred embodiment ofthe present invention uses a fin type body with dual double-gates. In afin type structure, the double gates are formed on each side of a thinfin shaped body, with the body being disposed horizontally between thegates.

While the invention has been particularly shown and described withreference to an exemplary embodiment using a fin type double gated fieldeffect transistor, those skilled in the art will recognize that thepreferred embodiment can be applied to other types of double gatedtransistors, and that changes in implementation details may be madetherein without departing from the spirit and scope of the invention.For example, it will also be understood by those skilled in the art thatthe invention is applicable to different isolation technologies (e.g.,LOCOS, recessed oxide (ROX), etc.), well and substrate technologies,dopant types, energies and species. It will also be understood that thespirit of the invention is applicable to other semiconductortechnologies (e.g., BiCMOS, bipolar, silicon on insulator (SOI), silicongermanium (SiGe).

1. A method for forming a semiconductor device, the method comprisingthe steps of: a) providing a semiconductor substrate; b) patterning thesemiconductor substrate to provide a body, the body having a first sideand a second side; c) forming a first double gate, the first double gatehaving a first portion adjacent to the body first side and a secondportion adjacent to said body second side; d) forming a second doublegate, the second double gate having a first portion adjacent to the bodyfirst side and a second portion adjacent to the said body second side,the first portion of the first double gate having a surface that iscoplanar with a surface of the first portion of the second double gate,the second portion of the first double gate having a surface that iscoplanar with a surface of the second portion of the second double gate;e) forming an insulating spacer between the first double gate and thesecond double gate.
 2. The method of claim 1, wherein the step ofpatterning the semiconductor substrate to provide a body comprisespatterning the semiconductor substrate to form a fin type body.
 3. Themethod of claim 1, wherein the semiconductor substrate comprise asilicon-on-insulator layer and wherein the step of patterning thesemiconductor substrate to provide a body comprises patterning thesilicon-on-insulator layer.
 4. The method of claim 1, further comprisingthe steps of forming a contact on the first double gate to provide afirst control signal and a contact on the second double gate to providea second control signal.
 5. The method of claim 1, wherein the bodycomprises a fin type body, and wherein the fin type body has a widthnarrow enough to insure a hilly depleted channel during operation of thesemiconductor device.
 6. The method device of claim 1, wherein the firstside of the body and the second side of the body are on opposite sidesof the body.
 7. The method device of claim 1, wherein the first doublegate is adapted to receive a first control signal and the second doublegate is adapted to receive a second control signal.
 8. A method forforming a semiconductor device, the method comprising the steps of: a)providing a semiconductor substrate; b) patterning the semiconductorsubstrate to provide a body, the body having a first side and a secondside; c) forming a first double gate, the first double gate having afirst portion adjacent to the body first side and a second portionadjacent to said body second side; d) forming a second double gate, thesecond double gate having a first portion adjacent to the body firstside and a second portion adjacent to the said body second side, whereinthe step of forming an insulating spacer between the first double gateand the second double gate comprises forming a first insulating spacerbetween the first portion of the first double gate and the first portionof the second double gate, and forming a second insulating spacerbetween the second portion of the first double gate and the secondportion of the second double gate.
 9. The method of claim 8, wherein thefirst insulating spacer and second insulating spacer are formed offsetsuch that the first insulating spacer and second insulating spacer arenot inline across the body.
 10. The method of claim 8, wherein the bodycomprises a fin type body, and wherein the fin type body has a widthnarrow enough to insure a fully depicted channel during operation of thesemiconductor device.
 11. The method of claim 8, wherein the first sideof the body and the second side of the body are not on opposite sides ofthe body.
 12. A method for forming a transistor, comprising: a) forminga fin body on a substrate, the fin body having a first vertical edge anda second vertical edge, and a first end and a second end; b) forming asource at the first end of the fin body, and forming a drain at thesecond end of the fin body; c) forming a first gate structure adjacentthe transistor body first vertical edge and second vertical edge, thefirst gate structure approximate to the source; d) forming a second gatestructure adjacent the transistor body first vertical edge and secondvertical edge, the second gate structure approximate to the drains; e)forming a spacer between the first gate structure and the second gatestructure, wherein the portion of the spacer adjacent to the firstvertical edge is offset from the portion of the spacer adjacent to thesecond vertical edge.
 13. A method for forming a transistor, comprising:a) forming a fin body on a substrate, the fin body having a firstvertical edge and a second vertical edge, and a first end and a secondend; b) forming a source at the first end of the fin body, and forming adrain at the second end of the fin body; c) forming a first gatestructure adjacent the transistor body first vertical edge and secondvertical edge, the first gate structure approximate to the source; d)forming a second gate structure adjacent the transistor body firstvertical edge and second vertical edge, the second gate structureapproximate to the drain, wherein a portion of the first gate structureadjacent to the first vertical edge of the transistor body is directlyopposite a portion of the second gate structure adjacent to the secondvertical edge of the transistor body.
 14. The method of claim 13,wherein the first gate structure adjacent to the first vertical edge ofthe transistor body has a work function which is more attractive todominant charge carriers of the transistor compared to that of the firstgate structure adjacent to the second vertical edge of the transistorbody.
 15. The method of claim 14, wherein the second gate structureadjacent to the second vertical edge of the transistor body has a workfunction which is more attractive to the dominant charge carriers of thetransistor compared to that of the second gate structure adjacent to thefirst vertical edge of the transistor body.
 16. A method for forming atransistor, comprising: a) forming a fin body on a substrate the finbody having a first vertical edge and a second vertical edge, the firstend and a second end; b) forming a source at the first end of the finbody, and forming a drain at the second end of the fin body; c) forminga first gate structure adjacent the transistor body first vertical edgeand second vertical edge, the first gate structure approximate to thesource; d) forming a second gate structure adjacent the transistor bodyfirst vertical edge and second vertical edge, the second gate structureapproximate to the drain, wherein an edge of the first gate structureadjacent to the first vertical edge of the transistor body is opposite,and displaced loss than the body thickness from an edge of the secondgate structure adjacent to the second vertical edge of the transistorbody.
 17. The method of claim 16, wherein the fin body has a widthnarrow enough to insure a fully depleted channel during operation of thetransistor.
 18. The method of claim 16, wherein the first side of thefin body and the second side of the fin body are on opposite sides ofthe fin body.